Andy Warhol's movie Screen Tests is exactly what it sounds like - a bunch of close-ups of people who drifted through the early Factory. They were asked to sit on a stool in front of a camera for what seemed like an eternity, without acting, dialogue, direction or doing anything at all. It sounds easy, almost boring - but in fact, when I did it, it was quite intense.

Cornell University had sent my class to Manhattan on a field trip to visit famous artists' studios, like Rauschenberg's and Oldenburg's, but Warhol's was as far as I got. Andy's studio was called the Factory and instead of the pristine white walls and bright lights of the other studios, it was dim and dirty looking, as if it were underground. The only light was reflected light coming off the tinfoil someone had pasted on the walls, making everything unreal and flickering.

Out of the gloom, poet Gerard Malanga walked up to me, ignoring my classmates and flashing his blond hair and insolent manner. I had met him before when he came up to Cornell to read his poetry, and I was happy to see him now. I felt very special indeed as he led me to a little silver-painted couch.

"Andy's doing something called Screen Tests," he whispered in my ear. "You just look into the camera for 15 minutes. Allen Ginsberg's already done one, he was great. Maybe I'll get Andy to do one of us together - we could set it up right now."

I watched my classmates file into the elevator as they started back to Cornell without me. They looked like foreigners now, as the steel doors of the elevator shut on their faces, and I forgot that I ever knew them.

I said yes, of course, knowing that the opportunity of being used by a famous artist in any way was better than sitting in art classes, which I was failing anyway.

Nothing happened for about an hour while I was ignored by the others who were busy ignoring each other. In the 1960s, it was important to be cool. Finally, I was led to the far end of the cavernous studio and told to wait - there was a lot of waiting here in the Factory - then Warhol walked over and people buzzed around while a camera was set up in front of my face.

Warhol did not introduce himself, which was fine with me. Later, I would learn that he was painfully shy and would rather eat ground glass than talk to a stranger. At the time I figured: he's the artist here - no need for formalities. Besides, I liked the freedom of anonymity. What I really liked was all the attention, but it didn't last very long. After turning on the camera, everybody left me there, alone. They just turned around and walked away, leaving me with a running camera pointed at my face. Next to the muzzle of a gun, the black hole of the camera is one of the coldest things in the world. I chose to ignore it, but what I couldn't ignore was the thought that this was all a joke, that there was no film in the camera and they were making bets at the other end of the Factory about how long I would sit there like an idiot. I recognised this rabid thought - paranoia - and, grabbing it, I quickly dragged it, kicking and screaming, to a closet in the lower left-hand corner of my brain.

Rather than spend my one and only screen test wondering if I was an asshole, I went the other way. Staring down the camera, I decided to take it all as seriously as a baptism, an ordeal one must pass in order to be inducted into the infamous underground.

Afterwards, like a new convert, I couldn't stop talking about what a genius Andy was; the way people's expressions changed in Screen Tests, making it a psychological study as their images cracked and their real personalities crept naked out of their eyeballs, the idea of conferring immortality upon unknowns - everyone's democratic little minute of fame - the deafening speechlessness of it all.

Like a medieval inquisition, we proclaimed them tests of the soul and we rated everybody. A lot of people failed. We could all see they didn't have any soul. But what appealed most of all to us - the Factory devotees, a group I quickly became a part of - was the game, the cruelty of trapping the ego in a little 15-minute cage for scrutiny.

I saw Salvador Dali take too flamboyant a pose for his test, and when the arm holding his cane collapsed, the upper lip holding his moustache twitched and drooped. I liked him better that way.

Now as I look back, I think we were the only ones excited by these games. What Warhol was doing is what any artist does - try to pin down the thing he finds most alluring and elusive, and for him, that was intimacy. In a society that frowns on staring as a breach of privacy, Screen Tests allows you to look for as long as you like into someone's eyes, as they look back at you. It is a voyeur's forbidden fantasy to have the tables turned by being the object of the gaze without its accompanying terror.

Looking at Screen Tests, it is hard not to pretend that these haunting faces do, in fact, finally recognise you as they stare out of their mysterious celluloid world. Of course, the person who loved watching these films the most - and did so over and over, while the rest of us ran to the other end of the Factory - was Warhol.

The black and white, the poor quality and cheap production, and the sparse documentary style increases the intimacy, making us feel that these faces have a life of their own, although they are frozen in another time. Unlike portraits or photos, they seem to be alive in their untouchable world. They breathe, they blink, all of which heightens a nostalgic sense of loss, both ours and theirs. They are masterpieces not of entertainment, but for contemplation.

Also, these early tests set the tone of Warhol's directing style, which was complete lack of direction. The 15-minute cage was lengthened to a reel as he refused to turn off the camera and we were left to our own rat-like devices until the reel ran out. Yes, yes, that was fun, more games and possible cruelty. But for Warhol, it was once again an effort to get close, to crawl under the defensive artifice of acting while remaining completely non-judgmental about what we did.

Some of the people in the films are dead now, and all of us no longer look as we did. The minute Screen Tests was finished, we all became ghosts trapped in the limbo of film.

 Screen Tests will be shown at the Andy Warhol & Sound and Vision season at the ICA, London SW1 (box office: 020-7930 3647), from July 28 to September 2. Mary Woronov's Swimming Underground: My Years in the Warhol Factory is published by High Risk Books, price £9.99.